1. Field of the Invention
The present invention relates generally to liquid crystal display devices and, more particularly, to liquid crystal display devices of the so-called active matrix type with switching elements being disposed in units of picture elements or xe2x80x9cpixels.xe2x80x9d
2. Description of the Related Art
Liquid crystal display devices are typically designed to perform a display operation by applying an electric field to liquid crystal molecules of a liquid crystal layer that is interposed between a pair of substrates to thereby permit liquid crystals to change in orientation or xe2x80x9calignmentxe2x80x9d direction and then utilizing resultant optical changes of the liquid crystal layer occurred due to such alignment direction changes.
One typical prior known active-matrix type liquid crystal display device is the one of the twisted nematic (TN) type, wherein the direction of an electric field being applied to its liquid crystal layer is set at a specific direction that is substantially at right angles to surfaces of the substrates with the liquid crystal layer sandwiched therebetween for achievement of the intended display operation by utilization of optical rotary polarization of the liquid crystal layer.
On the other hand, liquid crystal display devices of the in-plane switching (IPS) scheme type are also proposed until today, which are designed to employ a comb-shaped electrode while letting the direction of an electric field applied to liquid crystals be substantially parallel to the substrate surfaces to thereby perform displaying by use of double refractivity of the liquid crystals. Typical ones of such devices have been disclosed, for example, in Japanese Patent Publication No. 21907/1988, U.S. Pat. No. 4,345,249, WO91/10936, and Japanese Patent Laid-Open No. 160878/1994.
This IPS scheme is distinguishable over the traditional TN scheme in that the former offers advantages including but not limited to wider viewing angles and lower load capacitances. Presence of such technical superiority permits the IPS scheme to be adaptable for use with new types of active-matrix liquid crystal display devices which are widely used in place of the TN scheme type ones.
In the IPS scheme, as apparent from M. Oh-e, M. Yoneda and K. Kondo, Journal of Applied Physics, Vol. 82, No. 2, 1997 at pp. 528-535, it is possible to almost perfectly realize the intended in-plane switching operation in cases where liquid crystals are designed to have negative polarity dielectric anisotropy, rather than positive dielectric anisotropy.
Currently available IPS liquid crystal display devices employ optically opaque metal comb-shaped electrodes of stripe shape as provided within the surface of one of the pair of substrates stated supra.
In recent years, another type of IPS scheme has been proposed which employs comb-shaped electrodes made of a transparent conductive material such as indium tin oxide (ITO) in place of opaque metal electrodes, which electrodes are laid out at selected pitches that are less in value than those used in the prior art IPS scheme, and further uses a chosen liquid crystal material whose dielectricity anisotropy is negative in polarity to thereby make it possible by use of only electric fields as created at edge portions of a comb electrode to cause all the liquid crystals existing on or over this transparent comb electrode to offer the required alignment changeability, thus improving both the optical transmissivity and aperture ratio thereof.
The above-noted proposal is disclosed, for example, in S. H. Lee, S. L. Lee and H. Y. Kim, xe2x80x9cAsia Display,xe2x80x9d 1998 at pp. 371-374 and also in S. H. Lee, S. L. Lee, H. Y. Kim and T. Y. Eom, xe2x80x9cSID Digest,xe2x80x9d 1999, pp. 202-205.
It is also reported in the above-identified technical documents that with the IPS scheme using in combination certain liquid crystal material of negative dielectricity anisotropy and short-pitch transparent comb electrodes, the transmissivity near in value to that in the TN scheme becomes available while retaining wide view-angle characteristics equivalent to those in the IPS scheme.
A liquid crystal display device of the type employing the above-discussed technology is arranged to comprise a plurality of gate lines (gate lead wires) and multiple drain lines (drain leads) formed on or over a substrate, and also to comprise switching elements"" (typically, thin-film transistors, as the rest of the description as will be presented below assumes the use of such thin-film transistors) at cross-over points or xe2x80x9cintersectionsxe2x80x9d of the gate and drain lines, wherein more than one common electrode and pixel electrodes driven by the switching elements are disposed adjacent to each other.
A respective one of the thin-film transistors has its gate electrode formed of part of a gate line associated therewith, a drain electrode extended from its associative drain line, the drain electrode overlying the gate electrode with a semiconductive layer (amorphous silicon or xe2x80x9ca-Sixe2x80x9d layer) being interposed therebetween, and a source electrode for electrical connection to a pixel electrode. Note here that although the drain electrode and the source electrode are functionally interchangeable during an operation of the thin-film transistor, the following explanation will be given fixedly as shown in drawings to be presented later.
FIG. 17 is a diagram showing an enlarged plan view of a main part of a thin-film transistor in one exemplary IPS scheme liquid crystal display device. In FIG. 17, xe2x80x9cGLxe2x80x9d is used to designate a gate line; DL indicates a drain line; ASI denotes a semiconductor layer (amorphous silicon gr xe2x80x9ca-Sixe2x80x9d layer, also called a-Si island in some cases); PX denotes a pixel electrode; and CT denotes a common electrode. The element SD1 is a source electrode, whereas the element SD2 is a drain electrode. In this liquid crystal display device, the pixel electrode PX and common electrode CT are disposed over a thin-film transistor substrate in such a manner that these elements are in close proximity in position relative to each other. The source electrode SD1 and pixel electrode PX of the thin-film transistor are connected together via a through-hole TH.
In addition, FIG. 18 is a main part plan view diagram pictorially depicting a thin-film transistor portion in another example of the IPS liquid crystal display device. In FIG. 18 the same reference characters as those used in FIG. 17 designate the same functional parts or components. In this liquid crystal display device, the pixel electrode PX is formed directly on the thin-film transistor substrate, whereas the common electrode CT is formed to overlie the pixel electrode PX with a dielectric layer interposed therebetween. The source electrode SD1 is formed at the same layer level as the pixel electrode PX. The source electrode SDI of the thin-film transistor is connected to the pixel electrode PX via a through-hole TH.
Another structure is available, which is designed so that the common electrode CT is formed directly on the thin-film transistor substrate while letting the pixel electrode PX be formed thereover with a dielectric layer sandwiched therebetween. Still another one is also under consideration, which is arranged so that the shape of at least one of the pixel electrode PX and common electrode CT is bent or flexed in either a longitudinal direction or lateral direction.
With any one of the liquid crystal display devices shown in FIGS. 17 and 18 also, the semiconductor layer ASI to be formed over the gate line GL is oversized and extruded from the gate line GL at portions underlying the source electrode SD1 (i.e. the portions surrounded by circles xe2x80x9cAxe2x80x9d in FIGS. 17-18). When back-light rays reach and fall on such extruded portions of this semiconductor layer ASI, what is called the photoconduction currentxe2x80x94also known as photoconductivity current in some casesxe2x80x94might be generated, resulting in creation of current leakage at the thin-film transistor, or alternatively a potential decrease occurs in signal hold/retention voltage. Generally this xe2x80x9cphotoconxe2x80x9d current is derived due to, for example, an increase in conductivity of the semiconductor and/or the so-called intrinsic photoconductivity of the semiconductor which can take place due to the fact that rays of light incident on the semiconductor go beyond the forbidden band (band gap) of the semiconductor causing charge carriers to be produced inside thereof.
In addition, since contact portions (those surrounded by circles xe2x80x9cBxe2x80x9d in FIGS. 17-18) of the source electrode SD1 and the semiconductor layer ASI""s sidewalls are positionally adjacent to the thin-film transistor""s channel section (part xe2x80x9cCxe2x80x9d in FIGS. 17-18), hole injection can occur resulting also in creation of current leakage at the thin-film transistor and/or in signal retention voltage deterioration.
Furthermore, either the source electrode (or part for use as the source electrode of pixel electrode PX) SD1 or drain electrode as formed over the semiconductor layer ASI is such that electrical resistivity increases and/or open-circuiting would readily take place due to creation of cracks as resulted from the presence of a stair-step like difference or convexo-concave surface configuration of certain portions (edges of the source electrode SDI as surrounded by circles xe2x80x9cAxe2x80x9d or xe2x80x9cBxe2x80x9d in FIGS. 17-18) at which the electrode gets over the semiconductor layer ASI.
It should be noted that the above-discussed phenomena will also occur in TN type liquid crystal display devices, wherein the photocon current generation, thin-film transistor leak current production and/or signal retention voltage reduction might affect resultant brightness or illuminance in such liquid crystal display devices, which leads to a decrease in quality of on-screen display images. Additionally cracking and/or open circuiting of source/drain electrodes can result in a decrease in manufacturing yield of products. Thus, a need is felt to avoid these problems faced with the above-noted devices.
It is therefore a primary object of the present invention to provide a new and improved liquid crystal display device capable of avoiding the problems to thereby offer increased illuminance and enhanced operation reliability.
For resolving the aforementioned problems, in a liquid crystal display device, the present invention arranges a semiconductor layer being situated under a source electrode of a thin film transistor over a gate line (a gate electrode of the thin film transistor) so as not to protrude from the gate line, or expands a distance between a channel portion of the thin film transistor (formed of a part of the semiconductor layer) and a side wall of the source electrode at a portion thereof getting over (getting onto) the semiconductor layer. According to at least one of these features, the photoconductive current is suppressed.
Namely, one example of liquid crystal display devices according to the present invention comprises: a pair of substrates at least one of which is transparent and between which a liquid crystal layer is interposed; a plurality of gate lines and a plurality of drain lines being formed on an inner surface of one of the pair of substrates; a thin film transistor being disposed at an intersecting portion of one of the plurality of gate lines and one of the plurality of drain lines; a pixel electrode being connected to the thin film transistor; and a counter electrode being disposed opposite to the pixel electrode, wherein an edge portion of a semiconductor layer situated under a source electrode of the thin film transistor is arranged within an edge of the gate line situated under the semiconductor layer.
Moreover in the aforementioned liquid crystal display device, a width of the semiconductor layer situated under the source electrode is broader than that of the source electrode.
Incidentally in the aforementioned liquid crystal display device, the semiconductor layer provided for the thin film transistor (the aforementioned semiconductor layer having a portion utilized for a channel of the thin film transistor) is separated from another semiconductor layer disposed at the intersection portion of the gate line and the drain line (e.g. a semiconductor layer other than the aforementioned semiconductor layer extending along the drain line), so as to prevent the photoconductive current generated at the latter semiconductor layer from flowing from the intersection portion into the thin film transistor.
A drain electrode of the aforementioned thin film transistor should branch off from the aforementioned one of the plurality of drain lines at a location outside a gate electrode of the thin film transistor (e.g. the gate line situated under the aforementioned former semiconductor layer utilized as a channel of the thin film transistor), cover a corner of the semiconductor layer at a side of the one of the plurality of drain lines, and extend over the semiconductor layer. In this structure, the semiconductor layer should have so-called two directional getting over portion where the drain electrode gets over (onto) an edge of the semiconductor layer in two different directions with respect to an extension direction of the drain electrode.
Furthermore, the semiconductor of the thin film transistor should comprise a portion having two-or-three different edges (sides) thereof under at least one of the drain electrode and the source electrode of the thin film transistor. Since the drain electrode or the source electrode gets over or onto these two-or-three different edges along an extension direction of thereof, the portion of the semiconductor layer is called a two directional getting over portion or a three directional getting over portion in accordance with a number of edges of the semiconductor layer under the drain electrode or the source electrode. The two-or-three directional getting over portion prevents the source electrode or the drain electrode from being cracked or broken.
Describing a liquid crystal display device according to the present invention from another viewpoint, the liquid crystal display device comprises:
a pair of substrates at least one of which is transparent and between which a liquid crystal layer is interposed;
a plurality of pixel regions being disposed in a matrix manner along a first direction and a second direction transverse to the first direction on an inner surface of one of the pair of substrates, each of which has a gate electrode, an insulating layer being formed over the gate electrode, a semiconductor layer being formed on the insulating layer, source and drain electrodes being formed on the semiconductor layer and spaced from one another, and a pixel electrode being connected to one of the source and drain electrodes;
a plurality of gate signal lines respective one of which is connected to the gate electrode of each of the pixel regions being arranged along the first direction; and
a plurality of video signal lines respective one of which is connected to another of the source and drain electrodes of each of the pixel regions being arranged in the second direction,
wherein the semiconductor layer is formed within a contour of the gate electrode in each of the pixel regions.
In the liquid crystal display device of this sort, each of the plurality of video signal lines are formed on another semiconductor layer being formed so as to be spaced from the semiconductor layer of each of the pixel regions, for example.
Moreover, applying the present invention may be applied to TN-type (Twisted Nematic-type) liquid crystal display device by providing a counter electrode on an inner surface of another of the pair of substrates so that each of the pixel electrodes is opposite to the counter electrode across the liquid crystal layer.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.